4 Answers
4

I believe that it is common for the CPU to reach 90°C under heavy load. However a simple video should not bump an i5 into that region. Depending on the footage and format you are playing back you might want to try a different player (vlc mplayerX). If you are playing back flash HD videos in your browser (especially multiple streams simultaneously), that behavior might be common (due to the bad flash performance on os x).

So much for the software.

Secondly you should take care of the outer temperature (e.g.is your computer in a very hot room) and even more importantly where your notebook is placed. e.g. does it sit on a carpet or any other good insulating soft cushiony surface that prevents heat from leaving your computer.

Thirdly you might want to take a look in the activity monitor and see whats using your cpu. (Maybe some background application e.g. spotlight indexing new pdfs)

Thanks for your answer. 1) this heppens when i watch video on youtube or video call in skype or everything which uses video. 2) it is not very hot in my room. macbook is placed on the table without any cloth under it. 3) there is nothing heppening bad in the activity monitor :(
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user193542Aug 19 '12 at 12:14

Try SMCFanControl, a free app for regulating fan speed. Your Mac only increases fan speed once core temperature hits a certain threshold. At this point, the mac is already hot, and the fan must work overtime to lower it. If you expect a high load, you can primitively increase fan speed, to avoid the sudden temperature spike.

I should give a warning that 99.9% of the users shouldn't have to download this to make their Mac work.
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Enrico SusatyoNov 1 '12 at 3:37

yeah, it's unfortunate. 99.9% of users will never need to either, because they don't use that much of their CPUs. I saw an app that lets you change the fan speed to cpu temp relationship curve, so that you can lower the threshold needed to increase fan speed. I think apple deliberately makes the fan be triggered at such a high temp to reduce noise for the 99% of people who won't sustain such a temperature for so long.
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XAleXOwnZXNov 1 '12 at 4:14

After further clarification, it seems that you're getting this high temperature from running videos on YouTube's Flash Player. Flash Player is infamous for its overall crapiness, especially its resource usage (especially CPU usage). One solution to this problem is to find solutions for watching online video (such as off YouTube) without using their Flash Player. Here are the two applications I use:

ClickToFlash is a free app that replaces Flash content with a gray placeholder, which only runs the Flash content if clicked on. It works on most YouTube videos, detects the video frame, and lets you play the video using the HTML5 video player, which uses almost no CPU at all. As a bonus, it prevents Flash ads from running (which can sometimes slip by AdBlock)

HUDTube (it's accompanying Safari and Chrome extensions) allows you to open videos in an external player (much like QuickTime, but with a togglable ability to keep it on top of all other windows). It's useful to play the few videos ClickToFlash doesn't pick up.

your answer does not provide any useful information about the question, you use a much newer mac and playing a game is not the same as watching a video, you can always check here apple.stackexchange.com/help/how-to-answer to see how to answer correctly
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dennismuysJan 21 at 8:13